Semiconductor diode with suppression of auger generation processes

ABSTRACT

A multi-layer Auger suppressed diode having at least two exclusion interfaces and at least two extraction interfaces. A specific embodiment has two composite contacts, each consisting of a heavily doped layer ( 3, 4 ) and a buffer layer ( 8, 9 ) of lower doped, high bandgap material sandwiched between the heavily doped layer and the active region ( 2 ) of the device.

This invention relates to a semiconductor diode structure which givesrise to improved performance at room temperatures.

Narrow gap semiconductors have hitherto found few applications aroundroom temperatures because the intrinsic carrier concentrations are sohigh that they mask doping concentrations and lead to very high thermalgeneration rates with high leakage currents, high noise and lowradiative efficiency in emitters. Therefore they are typically cooled.

In order to capitalise on the potentially very high speed and very lowpower dissipation of narrow gap devices, Auger Suppressed devices wereinvented (see for example Proc. SPIE, Infra-red Technology XI Vol 572(Aug 20, San Diego Calif.) 1085, pp. 123-132). By electronic means thecarrier concentrations in an active zone are reduced even at ambienttemperatures or above so that extrinsic behaviour is achieved.

This is done by sandwiching a low doped layer between two contactingzones with interfaces of special properties. The first zone forms anexcluding interface and might have high doping of the same type as theactive zone, high band gap, low doping same type or a combination ofboth features. The important feature of the first zone is that theminority concentration is very low so that in reverse bias (that whichdrives minority carriers in an active zone away from the interface)carriers are removed from the active zone without replenishment from thefirst zone. The interface between such a zone and the layer into whichminority carriers cannot pass (in this case the active layer) is knownin an excluding interface.

In cadmium mercury telluride (CMT), for example, at room temperaturethis phenomenon exists over a wide range of material parameters. It issufficient only that the minority carrier concentration in thecontacting zone is lower than in the active zone. Typical doping in theactive layer might be below 5×10¹⁵ p-type with the contact zone morethan 10¹⁷ p-type, with or without a band gap increase in the contactzone of several times kT.

The excluding layer may be several microns thick, sufficient to minimisethe in-diffusion of minority carriers from the biasing contact itself.

The active layer may be several microns thick. It is usual to make itnot much more than the diffusion length of the minority carrier in theactive zone, and preferably much less. For a p-type active layer, fivemicrons might be a typical value. For n-type doping, typical valueswould be much less (less than two microns). This aspect is addressed inpatent publication EP0401 352B1.

Suppression will occur to some degree, whatever the length of the activelayer. The active layer is terminated by a second contact zone, withdoping of opposite type as in a junction. Again, the lower minoritycarrier concentration in this final layer, the better, and similarspecifications apply to this as do to the first contact zone (exceptthat the doping is of the opposite type).

With the bias in the same direction as before, minority carriers arecaptured at the interface and cannot return (in this case because of theusual barrier which exists in a reverse biased junction). Minoritycarriers migrate to the junction partly under the influence of the biaselectric field, and partly by diffusion. Such an interface between twozones, which allows carriers to pass between zones in one direction butnot the other but the other is called an extraction interface. Not allthree layer devices have exclusion and extraction interfaces. If thedoping and band gap conditions are not appropriate, then the applicationof a reverse bias will result in depletion. The conditions required fordepletion for a PIN device are described in EP-A-0193 462.

The overall effect is that minority carriers are removed at theextracting contact and are not resupplied at the excluding contact. Theoriginal concentration of minority carriers is large: near the intrinsicconcentration. After the application of bias it can be very low,typically below 10¹³, and often much lower depending on doping and bias.This low concentration contributes insignificantly to the space chargebalance so that the removal of the vast proportion of the minoritycarriers is accompanied by a loss of a corresponding number of majoritycarriers leaving a space charge balance comprising minority carriers(very small) and majority carriers close in concentration to the ionizeddoping concentration in the active zone.

These concentrations are typical of the cooled state, the active zone isin an extrinsic condition and devices can be constructed whichcapitalise on this.

Leakage current is present. Some of this is due to residual thermal andoptical generation in the active zone. Although thermal generation ratesare vastly reduced (“Auger suppression” due to reduced carrierconcentrations) they are not reduced to zero. The more ideally are thedoping considerations specified above met, the lower this will be. Inaddition to the above, there are other contributions to the leakagecurrent.

At the extraction junction there can be current leakage because theminority carrier concentration in the extracting contact zone is not lowenough. If the doping at the interface is significantly graded, say over0.5 microns, there will be a region close to the junction of low dopingand therefore relatively high minority carrier concentration. Thermalgeneration will lead to leakage current in the normal manner of animperfect junction diode.

Similarly, at the excluding junction there may be a graded interfacewith a short region of low doping/low bandgap which does not satisfy thepreferred specification, leading to unwanted carrier generation andleakage.

A further effect is the Debye-screened spill-over of carriers from ahigh doped region to a low-doped region, effectively grading even thesharpest of metallurgical interfaces and introducing minority carriergeneration in the tail of such a graded interface.

Even more difficulties can occur because of co-located rapid changes ofdoping and band gap which can give rise to temporary interruptions inthe otherwise regular switches in the levels of the band edges(so-called ‘glitches’) which can impede the proper flow of devicecurrents.

According to this invention a diode comprises multiple epitaxial layerssemiconductor material including:

a first outer layer of heavily doped p-type material;

an active layer of lightly doped semiconducting material; and

a second outer layer of heavily doped n-type material,

characterised by the diode further comprising:

a first buffer layer of lightly doped p-type material; and

a second buffer layer of lightly doped n-type material;

the layers being arranged in a stack with the first buffer layer beingsandwiched between the active layer and the first outer layer andforming, when a reverse bias is applied, an extracting interface witheach, and the second buffer layer being sandwiched between the activelayer and the second outer layer and forming, when a reverse bias isapplied, an excluding interface with each.

Preferably the the active layer is n-type or p-type material.

More preferably both the first buffer layer and the second buffer layerhave doping concentrations that are close to or equal to the dopingconcentration in the active layer, the bandgaps of said active layer andbuffer layers being such that the minority carrier concentration in eachof said buffer layers is less than one tenth of the minority carrierconcentration in said active layer.

In a further preferred embodiment, the doping concentration in theheavily doped layers is greater than 2×10¹⁷ cm⁻³.

In a further preferred embodiment the doping concentration in the activelayer is less than 5×10¹⁶ cm⁻³.

Advantageously the semiconducting material is a cadmium mercurytelluride compound having the formula Hg_((1-x))Cd_(x)Te wherein 0<x<1.

Conveniently the transition between the heavily doped semiconductingmaterial and the lightly doped semiconducting material takes place overa distance of several microns.

Preferably the thickness of the active layer is less than or equal tothe diffusion length of the minority carder. More preferably thethickness of the active layer is less than 5 μm. Yet more preferably thethickness of the active layer is less than 2 μm.

Advantageously the diode further comprises a substrate that is incontact with the one of the outer layers.

Conveniently the substrate is in contact with the first outer layer.

In a further preferred embodiment the doping concentration in the activelayer is less than 5×10¹⁶ cm⁻³. The doping concentration each bufferlayer would be between that of the active layers and the heavily dopedlayer to which it is adjacent and preferably close to that of the activelayer.

Throughout this specification, the terms “extracting” and “excluding”,when applied to interfaces between layers in a device, should beconstrued as indicating the nature of the interface with respect to theactive layer of the device. Moreover although, in the specificembodiment presented, a p-type active layer is shown, this should not beseen as limiting: the invention is equally applicable to devices with ann-type active layer wherein the roles of the other n and p-type layerswould be reversed with respect to the example given.

The invention provides a device which is suitable for incorporation byhybridization, growth in situ or otherwise in a variety of electronicdevices, such as a hybridized detector or an active circuit element suchas a high speed FET.

The invention will now be described by way of non-limiting example withreference to FIG. 1 which shows a schematic representation of an AugerSuppressed photodiode having a device structure which is typical of theprior art and FIG. 2 which shows a schematic representation of aphotodiode having a device structure which is typical of the invention.

Referring to FIG. 1, a typical Auger suppressed device of the prior art1 comprises a multi-layer structure having a lightly dopedactive(n-type) layer, 2 a heavily doped layer 3 of n-type materialforming and excluding interface with the active layer 2, and a heavilydoped, high band gap, layer 4 of p-type material forming an extractinginterface with the active layer 2.

The layer structure is grown on a suitable substrate 5 by standarddeposition techniques known to those skilled in the art, for examplechemical vapour deposition. Electrodes 6 facilitate the application of avoltage to the device.

Referring to FIG. 2, a device of the current invention 7 includes items2-6 performing a similar role to those in the device of FIG. 1. Inaddition, the device of the current invention comprises a lightly dopedp-type buffer layer 8 and a lightly doped n-type buffer layer 9 arrangedso that layers 9 and 3 form a composite contact zone as do layers 8 and4. Buffer layers 8 and 9 have a high bandgap relative to that of activelayer 2.

The reduced doping in the contact zone arising from the inclusion ofbuffer layer 9 reduces the spill-over and doping tail problems at theexcluding interface with active layer 2. Conveniently, the doping inlayer 9 could be made close to or equal to the doping in the activelayer 2. This would normally destroy the exclusion effect if the bandgap is the same. The band gap is therefore increased in layer 9. Inprinciple it could be increased so much that perfect exclusion occurs.In practice the change would need to be so great that material growthdifficulties would ensue and the inevitable tail might interfere withthe active zone by extending into it. Attention must be given to makingthe grading extremely sharp. A suitable increase in band gap might besuch as to reduce the intrinsic carrier concentration by 10 to 100times.

Further into the contact zone (the exact distance is not critical) agrade to high doping is made (layer 3). The grade may extend overseveral microns, this too is uncritical. In this wide gap material, theminority carrier concentration is already substantially smaller than inthe active region and in the new interface to high doping (as in theprior art) is able to act as an excluding interface in its own right.Application of the bias will cause the low-doped region between the highdoped zone and the active zone to be excluded efficiently, reducing theminority carrier concentration there much more (typically 10¹² or less).Thus, layers 9 and 3 form an extremely efficient composite contact forthe active zone, supplying no minority carriers.

In some circumstances of doping, the carrier concentration in thecontact can fall so low that extraction also occurs (loss of minoritycarriers by diffusion from the active zone).

In a similar fashion, the doping at buffer layer 8 is kept low but withincreased band gap (again such as to reduce the intrinsic carrierconcentration by typically 10 to 100 times) giving rise to extractioninterface between buffer layer 8 and active layer 2. The relativelyheavily doped layer 4 forms a second extracting interface with bufferlayer 8 to create a zone where the minority concentration falls evenlower and junction leakage is prevented.

The provision of high efficiency contact zones as described here ensuresthat the residual range of currents of these devices are almost totallydue to residual generation in the active current when biassed. Thisminimal current maximizes the ratio between maximum current (achievedalmost before onset of exclusion/extraction) and final current, andmaximizes the associated negative resistance effect.

Devices of the current invention can be fabricated using standardprocedures such as vapour phase epitaxy. Contacts can be standard forthe material employed.

Table 1 shows the structure and doping concentrations of a typicaldevice of the invention. The material of each layer of the device is ofthe family Hg_((1-x))Cd_(x)Te and is specified by the value of x. Inthis table, p and n indicate the type of doping in a material, thesuperscripts − and + indicate light and heavy doping respectively, theunderline indicates material of high band gap and π denotes the activelayer.

TABLE 1 Typical device of the Current Invention. Layer Doping xThickness (μm). p⁺ 3 × 10¹⁷ cm⁻³ As 0.35 4.0 p⁻ 1 × 10¹⁵ cm⁻³ As 0.351.5 π 1 × 10¹⁵ cm⁻³ As 0.18 2.5 n⁻ 1 × 10¹⁶ cm⁻³ I 0.28 3.5 n⁺ 3 × 10¹⁷cm⁻³ I 0.28 3.5

The structure shown in table 1 had a cut off wavelength of 9.5 μm and aminimum leakage current of 10 Am⁻².

What is claimed is:
 1. A diode comprising multiple epitaxial layers ofsemiconducting material including: a first outer layer (4) of heavilydoped p-type material; an active layer (2) of lightly dopedsemiconducting material; and a second outer layer (3) of heavily dopedn-type material, characterised by the diode further comprising: a firstbuffer layer (8) of lightly doped p-type material; and a second bufferlayer (9) of lightly doped n-type material; the layers (2, 3, 4, 8, 9)being arranged in a stack with the first buffer layer (8) beingsandwiched between the active layer (2) and the first outer layer (4)and forming, when a reverse bias is applied, an extracting interfacewith each, and the second buffer layer (9) being sandwiched between theactive layer (2) and the second outer layer (3) and forming, when areverse bias is applied, an excluding interface with each.
 2. A diodeaccording to claim 1 wherein the active layer (2) is n-type material. 3.A diode according to claim 1 wherein the active layer (2) is p-typematerial.
 4. A diode according to claim 1 wherein both the first bufferlayer (8) and the second buffer layer (9) have doping concentrationsthat are close to or equal to the doping concentration in the activelayer, the bandgaps of said active layer (2) and buffer layers (8, 9)being such that the minority carrier concentration in each of saidbuffer layers (8, 9) is less than one tenth of the minority carrierconcentration in said active layer (2).
 5. A diode according to claim 1wherein the doping concentration in the heavily doped material isgreater than 2×10¹⁷ cm⁻³.
 6. A diode according to claim 1 wherein thedoping concentration in the active layer is less than 5×10¹⁶ cm⁻³.
 7. Adiode according to claim 1 wherein the semiconducting material is acadmium mercury telluride compound having the formula Hg_((1-x))Cd_(x)Tewherein 0<x<1.
 8. A diode according to claim 1 wherein the transitionbetween the heavily doped semiconducting material and the lightly dopedsemiconducting material takes place over a distance of several microns.9. A diode according to claim 1 wherein the thickness of the activelayer (2) is less than or equal to the diffusion length of the minoritycarrier.
 10. A diode according to claim 9 wherein the thickness of theactive layer (2) is less than 5 μm.
 11. A diode according to claim 10wherein the thickness of the active layer (2) is less than 2 μm.
 12. Adiode according to claim 1 wherein the diode further comprises asubstrate (5) that is in contact with the one of the outer layers (3,4).
 13. A diode according to claim 12 wherein the substrate (5) is incontact with the first outer layer (4).
 14. A diode comprising multipleepitaxial layers of semiconducting material including: a first outerlayer of heavily doped p-type material; an active layer of lightly dopedp-type semiconducting material; a second outer layer of heavily dopedn-type material; a first buffer layer of lightly doped p-type material;and a second buffer layer of lightly doped n-type material; wherein thelayers are arranged in a stack with the first buffer layer beingsandwiched between the active layer and the first outer layer andforming, when a reverse bias is applied, an excluding interface, and thesecond buffer layer being sandwiched between the active layer and thesecond outer layer and forming, when a reverse bias is applied, anextracting interface.
 15. A diode according to claim 14, wherein boththe first buffer layer and the second buffer layer having dopingconcentrations that are close to or equal to the doping concentration inthe active layer, the bandgaps of the active layer and the buffer layersbeing such that the minority carrier concentration in each of the bufferlayers is less than one tenth of the minority carrier concentration inthe active layer.
 16. A diode according to claim 14, wherein the dopingconcentration in the heavily doped material is greater than 2×10 ¹⁷ cm⁻³ .
 17. A diode according to claim 14, wherein the doping concentrationin the active layer is less than 5×10 ¹⁶ cm ⁻³ .
 18. A diode accordingto claim 14, wherein the semiconducting material is a cadmium mercurytelluride compound having the formula Hg_((1-x)) CD _(x) Te wherein0<x<1.
 19. A diode according to claim 14, wherein the transition betweenthe heavily doped semiconducting material and the lightly dopedsemiconducting material takes place over a distance of several microns.20. A diode according to claim 14, wherein the thickness of the activelayer is less than or equal to the diffusion length of the minoritycarrier.
 21. A diode according to claim 20, wherein the thickness of theactive layer is less than 5 μm.
 22. A diode according to claim 21,wherein the thickness of the active layer is less than 2 μm.
 23. A diodeaccording to claim 14, wherein the diode further comprises a substratethat is in contact with one of the outer layers.
 24. A diode accordingto claim 23, wherein the substrate is in contact with the first outerlayer.
 25. A diode comprising multiple epitaxial layers ofsemiconducting material comprising: a first outer layer of heavily dopedp-type material; an active layer of lightly doped semiconductingmaterial; a second outer layer of heavily doped n-type material; a firstbuffer layer of lightly doped p-type material; and a second buffer layerof lightly doped n-type material, wherein the layers are arranged in astack with the first buffer layer being sandwiched between the activelayer and the first outer layer and the second buffer layer beingsandwiched between the active layer and the second outer layer, and whenthe active layer is n-type semiconducting material and a reverse bias isapplied, the first buffer layer forms an extracting interface with boththe active layer and the first outer layer and the second buffer layerforms an excluding interface with both the active layer and the secondouter layer, and when the active layer is p-type semiconducting materialand a reverse bias is applied, the first buffer layer forms an excludinginterface with both the active layer and the first outer layer and thesecond buffer layer forms an extracting interface with both the activelayer and the second outer layer.
 26. A diode according to claim 25,wherein the active layer is n-type material.
 27. A diode according toclaim 25, wherein the active layer is p-type material.
 28. A diodeaccording to claim 25, wherein both the first buffer layer have dopingconcentrations that are close to or equal to the doping concentration inthe active layer, the bandgaps of said active layer and buffer layersbeing such that the minority carrier concentration in each of saidbuffer layers is less than one tenth of the minority carrierconcentration in said active layer.
 29. A diode according to claim 25,wherein the doping concentration in the heavily doped material isgreater than 2×10 ¹⁷ cm ⁻³ .
 30. A diode according to claim 25, whereinthe doping concentration in the active layer is less than 5×10 ¹⁶ cm ⁻³.
 31. A diode according to claim 25, wherein the semiconducting materialis a cadmium mercury telluride compound having the formula Hg_((1-x)) CD_(x) Te wherein 0<x<1.
 32. A diode according to claim 25, wherein thetransition between the heavily doped semiconducting material and thelightly doped semiconducting material takes place over a distance ofseveral microns.
 33. A diode according to claim 25, wherein thethickness of the active layer is less than or equal to the diffusionlength of the minority carrier.
 34. A diode according to claim 33,wherein the thickness of the active layer is less than 5 μm.
 35. A diodeaccording to claim 34, wherein the thickness of the active layer is lessthan 2 μm.
 36. A diode according to claim 25, wherein the diode furthercomprises a substrate that is in contact with the one of the outerlayer.
 37. A diode according to claim 36, wherein the substrate is incontact with the first outer layer.